A thin film electroluminescence (hereinafter abbreviated as TFEL) element is a totally solid light-emitting element and is widely used as a display of a personal computer or a workstation taking advantage of its excellent visibility and high speed of displaying as well as the characteristics as a flat panel display in weight and thickness. A TFEL element typically has the structure shown in FIG. 1, called a double insulating layer structure. In more detail, a transparent conducting film comprising a solid solution of SnO.sub.2 and In.sub.2 O.sub.3 (ITO), ZnO:Al, etc. is laminated on transparent substrate 1, such as a glass substrate, and processed by photolithography and etching to form light-transmitting electrode 3 in stripes. Insulating layer 5 comprising SiO.sub.2, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5, Si.sub.3 N.sub.4, or a combination thereof is formed thereon by electron beam deposition, sputtering, etc. On insulating layer 5 is formed light-emitting layer 7 comprising a sulfide, e.g., ZnS, CaS or SrS, as a matrix activated with an adequate amount of a rare earth element or a transition element as a luminescence center by electron beam deposition, sputtering or chemical vapor deposition (CVD). Insulating layer 9 similar to insulating layer 5 is further formed on light-emitting layer 7, and electrode 11 comprising Al, Au, ITO, etc. is provided on insulating layer 9 in direction perpendicular to electrode 3. Electroluminescence (EL) occurs on applying an alternating voltage between crossing electrodes 3 and 11.
Conventional EL elements use a sulfide phosphor exemplified by ZnS as a material of a light-emitting layer. However, since a sulfide is instable against humidity and oxidation, a passivation for protecting an EL element against the outside is required. It is also pointed out that the light-emitting layer is gradually oxidized by the oxide used as an insulating layer to reduce the working life of the EL element.
On the other hand, oxide phosphors have been put to practical use in cathode ray tubes, plasma displays, fluorescent display tubes, and fluorescent tubes. Oxide phosphors are chemically more stable than sulfide phosphors and generally have relatively high resistance to moisture. A number of oxide phosphorus have been proposed to date.
Precedent patents relating use of an oxide phosphor in the light-emitting layer of a TFEL element include JP-A-63-232295, the term "JP-A" as used herein means an "unexamined published Japanese patent application") concerning an MIM structure (metal/oxide/oxide phosphor/metal) in which a light-emitting layer of an oxide phosphor is formed by sputtering to obtain emission of red light, green light or blue light; JP-A-64-67894 in which a fluorescent substance for a cathode ray tube which emits white light is prepared by electron beam deposition or radiofrequency sputtering, and a filter is provided for obtaining a color display; JP-A-1-213990 in which Ta.sub.2 O.sub.5 is formed by electron beam deposition, and luminescence from the traps due to oxygen defect is utilized for blue light emission, JP-A-2-297894 in which an alkaline earth metal oxide activated with a rare earth element is combined with an oxide insulating layer in an attempt to reduce the driving voltage, and JP-A-3-280394 in which a chain ion of CaWO.sub.4, etc. is used as a light-emitting layer.
However, many of the TFEL elements using an oxide in the light-emitting layer are considerably inferior in luminance to those using a sulfide. The reason seems to reside in difficulty in preparing a thin film of a highly crystalline oxide satisfying a stoichiometric composition ratio because many oxide phosphors have a complicated chemical composition ratio. Further, electron beam deposition and sputtering involve problems of cost and performance. That is, the resulting thin film tends to suffer from composition change due to deterioration of the target itself or defects due to scattering of clusters during thin film formation; large-sized apparatus is required because of involvement of a vacuum system; and preparation of an EL element of wide area is difficult.
Conventional AC-driven TFEL elements generally need high voltage for driving, requiring an expensive, high-pressure-resistant drive IC. It has therefore been demanded to establish a technique for reducing the driving voltage.